Maintaining mitochondrial function is critical for everyday operation of the heart. Proper mitochondrial function requires maintaining regulated and selective permeability of the mitochondrial inner membrane to ions and metabolites. Mitochondrial permeability transition occurs when the inner membrane loses its selective permeability by opening of a large-conductance nonselective channel, the permeability transition pore (PTP). High concentrations of mitochondrial Ca2+ and reactive oxygen species (ROS) are known to open PTP. PTP opening depolarizes mitochondria and causes mitochondrial swelling; thus, sustained opening of PTP leads to mitochondrial dysfunction and cell death, which is associated with many cardiovascular diseases including ischemia-reperfusion (I-R) injury and heart failure. Therefore, understanding how PTP is regulated has significant clinical value. It has long been known that increasing mitochondrial Ca2+ concentration opens PTP. More recently, mitochondrial dynamics mediated by fission and fusion have also been suggested to be involved in regulating PTP. However, the mechanisms by which Ca2+ and mitochondrial dynamics regulate PTP remain unknown. Our new findings show that increasing mitochondrial Ca2+ induces phosphorylation of cyclophilin D (CypD) through GSK-3? activation in mitochondria. Furthermore, we have found a transient opening of PTP (tPTP) that is distinct from conventional PTP and is regulated by mitochondrial dynamics proteins. Inhibition of the fission protein Drp1 increases this novel tPTP. Importantly, the inner membrane fusion protein OPA1 was found to be a critical factor for the novel tPTP. Although Drp1 inhibition is known to decrease pathologic PTP opening and reduce myocardial infarction in I-R, the mechanism of this fission inhibition-mediated protection is unknown. We postulate that the mitochondrial dynamics-mediated novel tPTP is a structurally distinct entity from conventional PTP, and thus in pathological conditions, can serve as a relief valve for excess matrix Ca2+ and proton gradient that induces ROS overproduction; as such, it could thereby prevent pathologic opening of PTP. Supported by our findings, the Central Hypothesis is that CypD phosphorylation induced by matrix Ca2+ is a key event for PTP opening, while mitochondrial dynamics regulates novel tPTP, and their interplay determines cardiac pathology outcomes. We will test this hypothesis by three specific aims: (1) to determine the mechanism of Ca2+-induced PTP opening, (2) to determine the mechanism of mitochondrial dynamics-regulated novel tPTP opening, and (3) to investigate the interplay between conventional PTP and novel tPTP in the pathological setting. The proposed studies will utilize advanced in vitro and in vivo cell and molecular biological approaches along with new fluorescence-based assays. Completion of the proposed studies will generate a new paradigm for the regulatory mechanisms of different forms of PTP and their functional interplay. The new findings will provide mechanistic basis for a new therapeutic strategy to decrease heart I-R injury and other cardiac pathology associated with PTP.